Phenylacetylene is a common impurity in styrene produced by dehydrogenation of ethylbenzene. The phenylacetylene is produced in the monomer synthesis step as excessive dehydrogenation of ethylbenzene takes place. Commercial grade monomers may contain up to about 150 ppm of phenylacetylene.
Low phenylacetylene feedstocks may be useful for anionic polymerizations. See Priddy et al., U.S. Pat. No. 4,389,517, which is incorporated herein by reference.
Phenylacetylene contained in said styrene-containing substances may be removed by selective hydrogenation. However, hydrogenation of vinyl group of styrene competes with the desired hydrogenation of the acetylenic group. A selective catalyst is desired for the reaction because the concentration of phenylacetylene is low compared to the styrene. Poor selectivity results in an undesired conversion of styrene to ethyl benzene or may result in the formation of polymers and tars.
The previous art of hydrogenating phenylacetylene in styrene feedstreams has primarily focused on the use of palladium-based hydrogenation catalysts. Although phenylacetylene levels can be reduced to nondetectable levels (&lt;2 ppm) with palladium-based catalysts, a large stoichiometric excess of hydrogen is necessary. This large excess of hydrogen leads to additional hydrogenation of styrene monomer to ethylbenzene thereby decreasing the selectivity of the catalyst.
For example, Japanese Kokai No. 84-216838 discloses a method for selectively hydrogenating phenylacetylene using catalysts, which contained at least one metal selected from a group comprising Pd, Pt, Ir, Rh and Ru and at least one element selected from a group comprising Pb, Bi, P, Sb, As, Te and S. Said method is not sufficiently selective for hydrogenating phenylacetylene.
U.S. Pat. Nos. 4,493,906 and 4,440,956 disclose methods and supported copper catalysts for selectively hydrogenating acetylenes in the presence of olefins and in particular diolefins. However, the processing conditions and hydrogenation requirements are different than for the phenylacetylene hydrogenation system. In the case of butadiene streams, hydrogenation temperatures of 50.degree. C. with approximately 250 psi hydrogen are required to maintain the necessary solubility of hydrogen in the feedstream. These somewhat harsh conditions lead to the formation of "green oils" which primarily result from polymerization products. These "green oils" decrease the lifetime of the catalyst.